There are many benefits to having circuitry that includes superconductive material. Superconductivity refers to that state of metals and materials in which the electrical resistivity is zero when the specimen is cooled to a sufficiently low temperature. The temperature at which a specimen undergoes a transition from a state of normal electrical resistivity to a state of superconductivity is known as the critical temperature ("T.sub.c "). The use of superconductive material in circuits is advantageous because of the elimination of resistive losses.
Until recently, attaining the T.sub.c of known superconducting materials required the use of liquid helium and expensive cooling equipment. However, in 1986 a superconducting material having a T.sub.c of 30 K was announced. See, e.g., Bednorz and Muller, Possible High T.sub.c Superconductivity in the Ba--La--Cu--O System, Z. Phys. B-Condensed Matter 64, 189-193 (1986). Since that announcement superconducting materials having higher critical temperatures have been discovered. Collectively these are referred to as high temperature superconductors (HTSs). Currently, superconducting materials having critical temperatures in excess of the boiling point of liquid nitrogen, 77 K (i.e., about -196.degree. C. or -321.degree. F.) at atmospheric pressure, have been disclosed.
HTSs have been prepared in a number of forms. The earliest forms were preparation of bulk materials, which were sufficient to determine the existence of the superconducting state and phases. More recently, thin films on various substrates have been prepared which have proved to be useful for making practical superconducting devices. More particularly, the applicant's assignee has successfully produced thin film thallium superconductors which are epitaxial to the substrate. See, e.g., Olson, et al., Preparation of Superconducting TlCaBaCu Thin Films by Chemical Deposition, Appl. Phys. Lett. 55, No. 2, 189-190 (1989), incorporated herein by reference. Techniques for fabricating and improving thin film thallium superconductors are described in the following patent and copending applications: Olson, et al., U.S. Pat. No. 5,071,830, issued Dec. 10, 1991; Controlled Thallous Oxide Evaporation for Thallium Superconductor Films and Reactor Design, U.S. Pat. No. 5,139,998, issued Aug. 18, 1992; In Situ Growth of Superconducting Films, Ser. No. 598,134, filed Oct. 16, 1990 now abandoned; and Passivation Coating for Superconducting Thin Film Device, Ser. No. 697,660, filed May 8, 1991 now abandoned, all incorporated herein by reference.
High temperature superconducting films are now routinely manufactured with surface resistances significantly below 500 .mu..OMEGA. measured at 10 GHz and 77 K. These films may be formed into circuits. Such superconducting films when formed as resonant circuits have an extremely high quality factor ("Q"). The Q of a device is a measure of its lossiness or power dissipation. In theory, a device with zero resistance (i.e., a lossless device) would have a Q of infinity. Superconducting devices manufactured and sold by applicant's assignee routinely achieve a Q in excess of 15,000. This is high in comparison to a Q of several hundred for the best known non-superconducting conductors having similar structure and operating under similar conditions.
A benefit of circuits including superconductive materials is that relatively long circuits may be fabricated without introducing significant loss. For example, an inductor coil of a detector circuit made from superconducting material can include more turns than a similar coil made of non-superconducting material without experiencing a significant increase in loss as would the non-superconducting coil. Therefore, a superconducting coil has increased signal pick-up and is much more sensitive than a non-superconducting coil.
Another benefit of superconducting thin films is that resonators formed from such films have the desirable property of having very high-energy storage in a relatively small physical space. Such superconducting resonators are compact and lightweight.
Although circuits made from HTSs enjoy increased signal-to-noise ratios and Q values, such circuits must be cooled to below T.sub.c temperatures (e.g. typically to 77 K or lower). In addition, it is desirable to directly interface or connect these cooled HTS circuits to other components or devices that might not be cooled. Most particularly, the signals from the cooled circuits often must be coupled to electronics at ambient temperatures.
Furthermore, low temperatures must be maintained when using cryo-cooled electronics and infrared detectors. In such situations an interface to couple signals between cooled and ambient temperatures is needed.
Generally, coaxial cables are used as signal interfaces. Coaxial cables are typically made of a central signal conductor (i.e., a center or inner conductor) covered with an insulating material (e.g., dielectric) which, in turn, is covered by an outer conductor. The entire assembly is usually covered with a jacket. Such a cable is "coaxial" because it includes two axial conductors that are separated by a dielectric core.
Although coaxial cables are generally used as signal interfaces, when connecting circuits which include HTS material, one end of the connecting coaxial cable might be in contact with a circuit cooled to 77 K, and the other end might be in contact with a device at a much higher temperature (e.g., room ambient temperature is about 300 K). Standard coaxial cables are not manufactured to operate under such conditions. When standard coaxial cables are used under such conditions, the signal losses may be quite high and the heat load by thermal conduction through the cable may be quite large.
Minimizing signal losses is important because the ability to transmit signals directly affects the sensitivity and accuracy of the devices. Insertion loss is a measure of such losses due to intermediary components. In equation form, if the output wattage of a circuit is P.sub.1 without intermediary components and P.sub.2 with intermediary components respectively, then the insertion loss L is given by the formula EQU L (dB)=10 log.sub.10 (P.sub.1 /P.sub.2)
Unless such losses are minimized, the benefits of using HTS or cryo-cooled materials may be lost.
Minimizing heat load is important because cryogenic coolers used to cool the HTS circuits generally have limited cooling capacity and are relatively inefficient. For example, the best cryocoolers currently available require the supply of approximately forty watts of power to a compressor to remove or lift approximately one watt of heat load. Therefore, it is preferable to limit heat load to 0.1 Watts or less.
Although minimizing heat load is important, it is also difficult. Standard coaxial cables are fabricated by extruding or swaging metal tubing (e.g. copper, gold, aluminum, stainless steel, or silver) over a dielectric (e.g., low-loss plastic materials, polyethylene materials, or Teflon.TM.). The thinnest extruded tubing of which applicant is presently aware is about 0.005 inches (about 0.127 mm) thick.
In addition, as described above, one of the advantages of using HTS materials in circuits for microwave systems is the elimination of resistive losses. However, the advantage of reduced resistive loss can only be fully exploited if reflection or return losses (i.e., losses due to mismatches in characteristic impedances of the components) are minimized. This is especially true for components to be used at high frequencies (e.g., mm wave).
A primary candidate for mismatch problems in circuits including HTS materials is the transition through which a coaxial cable is connected to the circuit. In general, HTS material and circuits containing same have optimal properties in a planar configuration. However, coaxial cable is cylindrically shielded. The transition between the planar circuit and the cylindrical cable may contribute significant reflection or return losses.
The circuit bonding process may also affect the geometry of the transition between the circuit and cable. Typical cables require a transition through which the cable may be attached or bonded to a circuit. Typical coaxial cable transitions use the inner conductor of the cable suspended in air (e.g., forming a pin) where the air acts as a dielectric. The suspended conductor may be inadvertently slightly bent during a typical bonding process. The geometry of the transition may suffer from unsatisfactory reproducibility problems because of the mechanical stability (or instability) of the pin. A further disadvantage occurs when the contact is wrapped around the inner conductor pin, unnecessarily increasing inductance.
In addition, the geometry of the transition between the circuit and cable will directly affect the ease of assembly of the device using such components. To maximize ease of assembly the packaging of HTS circuits that are cooled to cryogenic temperatures must include special input and output leads. As explained above, HTS circuits must be cooled to below T.sub.c. Generally, such cooling is achieved by holding the circuits in contact with the cold head of a cryocooler (e.g. enclosed in a vacuum dewar). To connect cooled circuits contained in a dewar interconnection points must be provided through a wall in the dewar. Such interconnections provide large thermal conduction paths for already inefficient cryocoolers.
The prior art has failed to provide a signal interface (including a transmission cable and cable-to-circuit transition) between cryogenic components and ambient-environment components for use in radio frequency applications of cold electronics and high temperature superconductors. The prior art has also failed to provide an interface and transmission cable which exhibit low thermal conduction and low electrical losses (e.g. impedance continuity and low reflection losses), and which work over a frequency range including UHF, microwave, and low millimeter-wave frequencies (e.g. up to 40 GHz). The prior art has further failed to provide such an interface which is also mechanically stable (and, therefore, reproducible) and relatively easy to use.